Recent experiments have shown that it is possible to maintain normal blood gases by ventilating at high frequencies with tidal volumes significantly less than the anatomic dead space. This finding could not have been predicted from our present knowledge of respiratory mechanics and gas exchange. The experiments in this application are designed: a) to define more precisely the conditions under which this type of high frequency ventilation is effective; b) to gain insight into the mechanism of gas exchange under these conditions; and c) to examine the short and long term effects of this high frequency ventilation on lung stability and cardiovascular function. We will set up and test several different ventilation systems capable of generating the required high frequencies. We will determine the effects of varying tidal volume and frequency, and the influence of mean airway pressure. We hypothesize that the mechanism is one of enhanced dispersion through the airways, and we will test this by measuring the effective dispersion coefficient for the whole lung and along different segments of the airway tree. The results will be compared to the predictions of theoretical models. The cardiovascular effects will be studied in animals chronically instrumented with pressure catheters and electromagnetic flow probes. The results of our experiments will lead not only to a better understanding of pulmonary gas exchange, but also to the development of ventilation systems for clinical use, which may eliminate the complications frequently associated with artificial mechanical ventilation.